Multiple scenes and material with pass index
This is because there is no thickness to the outside cube.
Your file is setup so that when the camera &quot;sees&quot; the face on the front it is replaced with the inside scene.
However, when the camera goes inside the cube, the camera can no longer &quot;see&quot; that face, so nothing is replaced with the inside scene:

Solution:
One way to fix this is to make the outside cube have an inside, by giving it some thickness. The easiest way to do that is with a Solidify modifier:

Delete the inset face on the front of the cube.

Add a solidify modifier in Properties &gt; Modifiers &gt; Add Modifier:



Now assign the door material to the inside faces:

Apply the Solidify modifier.

Select the inside faces and assign the &quot;door&quot; material to them:



Now your setup should work.
Frame 60 looks like this: